Quarq Accuracy

I've ridden one stretch of road enough to try to see how repeatable the Quarq CinQo is. It's the stretch from the bottom of Montebello to the school. So, we can play some games with numbers. Let's assume that I didn't change wheels (I probably put a set on that weighs ~1/2 lb lighter for some runs) and that my body weight and clothes were constant. Here are the results:

time	average power	deviation	kJ
00:14:09	284	101.91%	241.1
00:15:39	250	100.24%	234.8
00:12:28	326	101.78%	243.8
00:12:39	314	99.83%	238.3
00:13:33	286	98.21%	232.5
00:13:03	302	99.40%	236.5
00:12:37	311	98.70%	235.4

So, the mean is 237.5 kJ, and I've tabulated the deviation from this. It looks like the Quarq is well within its +/-2% accuracy spec, but of course, this spans a lot of time and probably many variations in weight and equipment. Oh yeah, I'm neglecting the confidence in the mean and such things that probably matter. I'm basically impressed that a device as rugged as this can give such repeatable data day after day.

OK, enough geeking out for a day.

Coast down estimation of drag and rolling resistance - Part 1

So I promised some geekery here. The goal was to do a simple test and try to estimate rolling resistance and drag coefficients. The simplest thing anyone can do is a coast down: you accelerate to speed, stop pedaling, and coast to a halt (or close to it.) So, to do the analysis, you first need to solve the equation that describes the motion:



where:
ρ is the density of air
c_rr is the coefficient of rolling resistance
C_dA is the drag coefficient multiplied by the frontal area of bike and rider
m is the mass of rider and bike
g is the acceleration due to gravity
v is the velocity of rider and bike

You can use separation of variables to get a rather gnarly looking expression that describes the coast down behavior of a rider in a constant position:



where I've defined:



I went out and did several coast-down tests in two positions (straight arm in drops, hands on tops,) and covering the same course forward and backward. The reason for this is that and slope will look like a change in rolling resistance, so if you take the average rolling resistance derived from the two directions, that should cancel out any error due to slope. It does NOT cancel out wind, so I did this at midnight, which is typically fairly wind-free. I downloaded the data from the powertap and used the nonlinear fitting function in Excel (blah :() to find values for c_rr, C_dA and t₀. I then used R to see if there was a statistical difference in rolling resistance and drag between the two positions I used, and to estimate the error in those values. Next time, I'll show some data.

Teetering on the edge

One of the challenges of training is getting close to the edge, but not falling off. The human body has a strange negative feedback mechanism. Do too much physical work and you have problems sleeping, and you slow down, making you think you have to do more work to compensate. I've overtrained before, but this year I've been pretty good (so far) at walking up to that edge and not going over it. This weekend was pretty close to it for me; after riding ~5 hours in 105 degree weather with a bunch of climbs on Saturday and three times up Mt Diablo on Sunday, I'm due for some rest.

A Rarity

My wife came with me to the race. It's probably the only road race worth going to to watch the race, though there may be others in cool places to visit. She grabbed this photo of me:



So I don't just make up stories about riding...

University Road Race

Ah, University. As a simpleton racer, this is a race I can actually train for. It goes up, then it goes down. I usually like that kind of thing. Things started in earnest, with Menso de Jong rolling off the front repeatedly, and Chris marking him early on in the race. The pack reeled them in several times before Menso finally broke the elastic and rode away from everyone in true pro MTBer style. Chris followed after him, but got reeled back in one last time. Somehow, I managed to remember to drink without him there to remind me, but from now on I'm gonna have "god, do I have to mother you?" stuck in my head in Chris's voice.

Like I said, I'm simple racer, so I followed some simple rules for the rest of the race: coast up to the front on the descent, drift back on the climb, and never stand on the kicker unless absolutely necessary. I closed gaps where necessary as attrition really set in after 10 laps, but basically sat in pretty comfortably thanks to all the work Chris was doing off the front.

With only Menso long gone off the front for the last few remaining laps, not too much was going on in the pack. I managed to drop my chain and had to do a pretty hard effort to chase back on, but even that was fairly uneventful. I repeated my simple plan and thought about what I was going to try to do on the last lap. Somehow, I got it in my head that it would be a fantastic idea to launch a sprint from the corner, about 2 minutes from the finish line. Unfortunately, a rider from Cyclepath had already cleared the group by the last corner when I wasn't paying attention, so it was a race for third. I attacked hard up the left side right after the corner, looked Chester from Platinum square in the eye and we were off. I led out for the last 1.5 min, then Chester came around me for 3rd near the line; I held my position for 4th. We almost caught the Cyclepath guy, so I can't say I made the wrong choice, just not the best. Glad I was able to hold on for so long given my early "sprint" start!

I stuck around to cheer on the 4/5 and women's fields, then had a great lunch in SC before heading home.

Legs are finally coming around, so I'm looking forward to at least Challenge and Everest later on this year.

Patterson Pass

Patterson Pass was my first race as a 3 last year. It's a hard man race, and I don't really belong to that category of rider. This year there was no wind so the climb was extremely selective. Being my first real race back in a long time, I hung in until the 3rd lap then got dropped as the heat and pace got to me. I soft pedaled to the top, caught a bunch of riders going back to the finish and slotted in at 12th. Nothing much else to report except that it was HOT. No water at the finish this year so I was parched by the time we stopped at the first gas station to BUY water. I'm not at all a believer in buying something that comes virtually for free from a tap, so it was a painful move for me to buy a liter of water and some V8 to replace all the salt that was all over my kit and face. Good stuff.

I got a new power meter. I have two sets of nice wheels, but since I'm a geek, I rarely use them so I can get power data from races. This, of course, being suboptimal, needed to stop, so when Quarq released their compact Cinqo, I jumped on it. I'd been saving the money since March, when I originally thought they were going to release it, so even though it's an expensive toy, it didn't hurt too badly. The first one broke on me during my crazy ride of the year (3x Diablo with Everest Challenge teammates) but Quarq's customer service came through in two big ways:

1) they answered email on a Saturday. They have this funny habit of apologizing for answering late, you know, because 2 hours is a long time to wait for help. I'm used to vendors taking forever, so I'm already impressed.
2) they overnighted a new crank to me. I barely missed using it. Awesome.

The Cinqo and powertap agree very well. I have Tukey mean-difference plots to prove it, but those who actually read this already know I'm a dork so I won't plot that here. I will, however, update you on how I measured my drag coefficient and rolling resistance in the next post.

Leesville

Yeah, so that was interesting. Woke up with a stiff back that didn't want to loosen up and beat it with a stiff carbon frame across some beautiful dirt roads in Northern California. Break a few parts, lose some water and finish on the most god-awfully boring roads I've ever seen and call it a day. Probably not doing this one again. Basically, after breaking my power meter and getting a rock in the knee, I sat up and rode it home.

Good to get out in a fast pack again, and the damage wasn't anything a soldering iron couldn't fix.